The present invention is directed to a method and apparatus for fast imaging of a part of a body subjected to an intense magnetic field known as an orientation field during a nuclear magnetic resonance (NMR). This type of imaging is currently meeting with increasing success in the medical field where the images produced serve as a diagnostic aid, especially in the diagnosis of cancer. The application of the disclosed embodiments of the invention however is not limited to this field. The embodiments can also be implemented, for example, in physical measurements using spectrometers.
In nuclear magnetic resonance imaging, an image of a slice of a body to be examined is obtained by subjecting the body in question and, especially, the part in which the slice is located, to a continuous, intense and homogenous magnetic field B0. Under the effect of this field B0, after a few instants, (within a few seconds), the magnetic moments of the particles of the body align their orientation with the direction of the magnetic field: this is why this field is known as an orientation field. If the magnetic moments of the particles of the body are then excited by an RF excitation oscillating at an appropriate frequency, it causes the orientation of the excited magnetic moments to get tipped or flipped.
At the end of the excitation, these magnetic moments tend to get realigned with the orientation field in a motion of natural precession known as free precession. During this precession motion, the particles radiate an electromagnetic de-excitation energy that can be measured. The frequency of the de-excitation signal, also called the NMR signal, is characteristic of the excited particles (in medicine, these particles are the hydrogen atoms contained in the water molecules disseminated throughout the human body) and of the force of the orientation field. The characteristics of the body are deduced from the processing of the measured signal.
The processing of the measured signal in order to extract an image gets complicated because all the particles of the body throughout the excited region re-emit a de-excitation signal at the end of the excitation. It is therefore important to distinguish the contributions, in the total NMR signal, of all the elementary regions (known as voxels) of the volume excited to reconstruct their distribution, ultimately to prepare the image. This distinguishing can be done only by performing a series of excitation-measurement sequences during each of which the NMR signals to be measured are encoded differently from one sequence to another. Since the encoding applied is known, the image can be reconstructed by pure imaging techniques, especially of the 2DFT type.
The measurement of the NMR signal actually relates to the amplitude of this signal. Indeed, given the modulation frequency around which the NMR signal is examined, all that can be hoped for as a measurement result is a measurement of the density, in the structures examined, of the specific particles (hydrogen) for which only one of the resonance frequencies is examined. Broadly speaking, at the end of a given period of time after the excitation, the greater this density, the stronger is the NMR signal. Indeed, this density does not act solely on the original amplitude of the NMR signal. In practice, in medicine, it is even assumed that all the regions of the body, from this viewpoint, make the same contribution to the NMR signal. However, the density acts fairly strongly on the damping, namely the relaxation, of this NMR signal. This damping is a complex damping: it represents an interaction known as the spin-lattice interaction of the excited particles (the proton of the hydrogen atom) with the surrounding matter and an interaction known as the spin-spin interaction of the protons with one another.
In a known model of the physical phenomena that come into play, it has been determined that the spin-lattice relaxation time, known as the time T1, corresponds to the time constant of an exponential regrowth (a reorientation) of the component, aligned with the orientation field (also called the longitudinal component) of the total magnetic moment at a concerned position of the body. The spin-spin relaxation time, known as T2, also corresponds to a time constant but in this case to an exponential decrease of the transverse component (orthogonal to the longitudinal component) of these magnetic moments. In an example that shall be referred to further below in the context of the description of an embodiment of the invention, reference shall be made to a time T1 of about 500 ms and above all a time T2 of about 100 ms: the concerned regions of the body will be mainly the head and more specifically the brain.
It is possible, in different types of series of imaging, to obtain the appearance of one relaxation phenomenon in preference to another. It is said then that images are made in T1 or T2 as the case may be. The essential parameter of NMR images brought into play in this case is then the repetition time TR that marks the periodicity of the sequences of the series of imaging sequences implemented.
It is known that it is possible to make use of the T2 image, with its characteristics of differentiation. In particular, it is known that, in the human brain, gray matter, white matter and to an even greater extent tumors, possess well-differentiated responses in T2. In practice, the NMR signal measured is never anything but a signal corresponding to the component orthogonal to the orientation field of the motion of precession of the flipped magnetic moments. Now it is known that, if the repetition time is in the range of a mean value of T1, the amplitude of this signal directly represents the contributions in T1 of the different parts of the body.
To make an image in T1, it is necessary to wait for a total regrowth of the magnetization (of its longitudinal component): the duration of the wait necessary between each sequence is about three or four times the duration of T1. At the end of this duration, leaving aside the concentration of the particles (which are not considered), it can be said that the first NMR signal measured is dependent only on the relaxation time T2. It is only if the repetition rate is too fast that the effect of the decrease of the NMR signal in T2 disappears in the face of the importance of the differentiation of the regrowths in T1.
At this stage in the explanation, there immediately appears one of the difficulties pertaining to the images in T2: they are long. In practice, they are about two or three times longer than the T1 images. For example, for a T2 image of a section of the head, a patient must undergo examination without moving for 17 minutes. To prevent the patient from moving, it becomes necessary to fix his head in a collar, which makes the NMR examination very distressing even if it is not harmful.
A known way of overcoming the drawbacks referred to consists in proposing a fast T2 image. In one example, the image may last about two to three minutes. At the same time, the disturbing effects of the contrast seen in T1 are avoided in this image. The goal in view is that the period of acquisition should not be too lengthy both for the patient and in terms of the economical use of the machine.
Recently, fast image acquisition methods known as fast spin echo or FSE type methods have been proposed. In practice, there are sequences known as pure FSE sequences, with typically 16 echoes and 16 acquisitions, repeated every four seconds. This gives 64 seconds of acquisition for one section. Other methods, known as single shot FSE or SSFSE methods, are described below. These SSFSE methods use sequences comprising an excitation of the magnetic moments of the protons, known as a flip, prompting a major flip of the magnetization, which is typically a 90° flip (whereas in the SSFP type methods the original flip was a small-angled flip) followed by a large number of spin echo excitation pulses (at 180°), known as refocusing pulses, very close to each another, typically separated from each other by a few milliseconds. Furthermore, phase-encoding gradients are applied between each of these refocusing pulses, and before the reading. These phase-encoding gradients vary in successive steps from one echo to another. The gradient pulses thus applied are furthermore compensated for in a subsequent gradient pulse before the next refocusing pulses. As regards the section select gradient pulses, the fact of centering them in time on the central date of the refocusing pulses gives rise to automatic compensation for these pulses. It can be shown that this is also the case for the read gradient pulses.
Typically, it is thus possible, during a single sequence of decrease of the signal in T2 (equal for example to about 400 ms), to acquire 128 echoes, each representing what happens in one line of the image. By thus applying read gradients during the reading, it is possible, after a single sequence, to acquire a full image in a very short period of time, equal to about T2 or a multiple of several times T2. Typically, each refocusing pulses may last 5 ms (to be properly selective), or 2.4 ms in being less selective. The measurement duration of the NMR signal, at the time of each echo, in taking 256 samples every 8 microseconds, last about two ms.
One presentation of this technique is given in U.S. Pat. No. 6,265,873, the contents of which are entirely incorporated herein by reference. The present invention is an improvement of the method described in this patent.
FIGS. 1a to 1g show the geometrical, theoretical and practical consequences of the refocusing excitations of the FSE type fast refocusing methods. They also show how the problems of signal quality were resolved in the above-mentioned patent. The problem resolved by the present invention is then explained. Specifically, the method disclosed in the patent referred to above, and in the present invention, comprises an additional encoding to be applied to the refocusing pulses in order to improve the quality of the NMR signal detected. Rather than encoding the amplitude of these refocusing signals, which results in complex problems of shape definition, it is the phase of these refocusing pulses that has been encoded. The phase is determined with respect to a synchronized date of evolution of the NMR phenomenon. In this way, a 3 dB improvement is produced in the level of the signal detected. It will be shown that this improvement makes it possible either to acquire the images even faster or, above all, to take account thereafter, in the images, the phenomena of movement by the patient, the phenomena of molecular diffusion and the phenomena of chemical shifts. These phenomena ultimately make it possible to measure the temperature of the patient's body through a measurement of the NMR signal itself. This was hitherto not possible with the FSE sequence.
FIGS. 1a to 1g are shown in the referential system rotating at f0, f0 being the free precession frequency, known as the Larmor frequency, of the magnetizations of the protons subjected to a magnetic orientation field Bo. This rotating referential system comprises an axis z oriented as the field Bo and axes x and y forming a plane xy in which, from one refocusing excitation to another, the orientations (or rather the measured components) of the excited magnetic moments are supposed to occur. At the outset (FIG. 1a), the magnetization of each proton is equal to Mo and is oriented along the direction z. During a first 90° excitation, corresponding to a flip around the axis y (FIG. 1b), the magnetization Mi is collinear with the axis x. The rotating reference system xyz is herein shown arbitrarily as rotating in the normal direction.
FIG. 1c shows the phase shifts of the components. These phase shifts are due to the inhomogeneities of the field Bo. These phase shifts are furthermore reinforced by the presence, in free precession, of superimposed field gradients. Thus a schematic view is given of components x1 and x2 in the plane xy. The components x1 correspond to protons precessing at a respectively lower frequency. These are slower protons. The components x2 correspond to the faster protons. The in-phase component of the orientation values of these protons continues to remain aligned with the axis x.
FIG. 1d shows the effects of a refocusing pulse about the axis x. These pulses about the axis x, which are perpendicular to the initial excitation which was about the axis y, correspond to excitations called CPMG (Carr-Purcell-Meiboom-Gill) type excitations. Typically, before the application of the 180° excitation, the components x1− and x2−, in accordance with their form shown in FIG. 1c, are turned over by an angle φ=180° into components x1+ and x2+ whose main characteristic is that they are now located in a position symmetrical with their initial position in relation to the magnetization in a phase oriented along the axis x. In a known way, at the end of a period of time, after the 180° pulse, equal to the time between the application of the 90° pulse and the application of the 180° pulse, the NMR signal presents all its components in phase along the axis x. It is again measurable.
However, the 180° refocusing pulse is not applied with perfect efficiency to all the protons examined. It appears that certain protons are subjected to a smaller-angle refocusing around the axis x. The protons concerned are those located on the front face and the rear face of the section. Indeed, the cases of 180° refocusing cannot be perfect in the section and zero outside the section. Otherwise, the duration of the 180° pulse would have to be very great (in theory infinite). In the transition zones, the refocusing is therefore imperfect. FIG. 1e shows a refocusing where φ is about 135°. In this case, rather than being located in the plane xy, as in FIG. 1d, the component x2− has been converted into a component x2+ possessing a significant component along the axis z. This significant component reduces the measurable component x′2+, shown in the plane xy. Nevertheless, this component x2+ gets phase-shifted after the application of the 135° pulse to which it has been subjected according to the drawing of FIG. 1e. 
This phase shift leads firstly to a subsequent resynchronization during which the NMR signal is again measurable (with a contribution that is all the same deteriorated but negligibly so for the component x2), and continues to get phase-shifted so that, during a subsequent 180° refocusing excitation, in FIG. 1f, it is again at x2−.
In short, it can be noted in FIG. 1f that, just before this subsequent refocusing pulse, the component x2− is in the position that is symmetrical to that of the component x2+ of FIG. 1e (with a same component along z). If it is then subjected to a same refocusing operation, with a same φ=135° amplitude smaller than 180°, as it was during the previous refocusing pulse, the component x2− gets converted into a component x2+ that is now located in the plane xy.
In other words, every other time, even if the pulses at 180° are imperfect, it seems that, if the 180° pulse is applied to an axis (x) perpendicular to the axis (y) about which the initial excitation pulse has been applied, there is a phase-shifting of all the magnetizations of the protons. This is corroborated in FIG. 2a where the y-axis indicates the modulus of the measurable NMR signal while the x-axis indicates the refocusing excitation number in the sequence (here there are 32 of them) for which the NMR signal is measured after standardization. It can be seen that, on the different curves, corresponding to 30° variations (180°, 150°, 120°, 90°, . . . ) in the refocusing pulse, the amplitudes of the NMR signals remain constant, even if they are of lower value if the flipping is imperfect.
By comparison, the FIG. 1g shows what happens when the refocusing pulse is done about the axis y, namely the axis about which the magnetic moments have flipped, given the initial 90° excitation pulse. The 180° refocusing has not been shown because, naturally, for the protons subjected to these 180° refocusing pulses (whatever the axis) the phenomenon is perfect in theory. There is a natural synchronization. It will furthermore be observed that the fact of prompting a flipping of the magnetic moments about the axis y rather than about the axis x amounts in fact, in this case, to effecting a temporal shift, equivalent to a delay of a quarter period (a period equal to 1/f0), in the application of the refocusing pulse.
FIG. 1g shows that a magnetization component x2− in the plane xy (before the application of a major refocusing pulse) gets flipped into x2+ after the application of the refocusing pseudo-pulse φ having a value below 180°. Here, we have only shown the case of the fastest protons so as not to burden the drawing. As in the above case, it can be seen already that the component x2+, after application of the refocusing pulse, has a component oriented along the axis z that is negative. It must also be noted that the component along x is now negative (whereas it remained positive earlier). Consequently, just before the application of the subsequent 180° refocusing pulse (which itself is also imperfect, but under the same conditions), the component x2+ has become x2+−, which is symmetrical to the plane xz of the component x2+.
If this component x2+− too undergoes a φ flip (smaller than 180°) identical to the previous φ, the component x2+− becomes a component x2++. This component x2++ then has the particular feature of also having a component oriented along the axis z while, according to the excitations called CPMG excitations of the previous figures, the component along z would have been reduced to zero. In the schematic example shown, the component along z is different from the component along z after the previous refocusing pulse. It has even risen in such a way that soon there is no longer any measurable NMR signal at the time of the resynchronization. This occurs in fact after a small number of echoes, four or five in practice, in any case fewer than ten.
This is shown schematically in FIG. 2b, which is presented so as to correspond to FIG. 2a, and is based on the same assumptions as this FIG. 2a. FIG. 2b shows what happens when the condition known as the CPMG condition (corresponding to FIGS. 1a to 1f) is not fulfilled. It is noted that, for different 180° flips, firstly the measurable NMR signal decreases very sharply with the rank of the echo. It is no longer even measurable after the tenth echo. Secondly, the NMR signal undergoes an alternating development whose period is all the greater as the divergence from the 180° value is low.
Considerations of this type have led to consider pulses not meeting the CPMG conditions as being unsuited to the production of measurable NMR signals. At least, it could be asserted that if the magnetization were not oriented along the axis x, the orientation would be analyzed as a combination of a component along the axis x (called a real axis) and a component along the axis y (called an imaginary axis). Using the FSE method under CPMG conditions actually amounts to destroying the imaginary component. Thus 3 dB are lost. Furthermore, owing to movements by the patient, increasing numbers of components are thus gradually eliminated. Indeed, with the patient's movements, the phase of the magnetizations gets shifted and the off-phase component of these shifted magnetizations is dampened. It will furthermore be noted that this dampened component does not disappear from the signal but reverberates in the image in the form of black dots that scramble this image.
To resolve this first problem, it has been proposed to eliminate one of the components of the signal by means of gradient echo pulses so that they disappear from this signal. However, the elimination of this component, dividing the useful signal by two, results in a fourfold increase, for identical signal-to-noise ratios, of the acquisition time for an image, and this is not acceptable.
It will be noted that this duration, which is limited by the absorption of energy in the patient, must be considered in the case of the acquisition of multiple-section images (for which the duration of acquisition is proportional to the number of sections) and of images of directions of molecular diffusion. It is sought, indeed, to acquire images of components of molecular diffusion in one direction and then another and so on and so forth. It can be shown that images in three cardinal directions are not as interesting as images acquired in a hundred directions. These images make it possible especially to measure the presence of the fibers (in which the diffusion is restricted and has a preferred direction). In particular, at the position of neurons of the brain, the depiction of a mean component (in a single direction) is of no interest as compared with knowledge of the numerous ramifications of the neurons. What must be done is to determine the numbers and orientations of these ramifications through the directions measured. The numbers of the acquisitions thus soon become a problem if each acquisition lasts too long. For example, 100 directions and 10 sections entail a 20-minute examination, even if each image is acquired within one second.
With the conditions known as CPMG conditions (90° flip and 180° refocusing excitations on the mutually perpendicular axes y and x, and the activation of these excitations in phase), it was observed with FIG. 1g that a component of the NMR signal disappeared rapidly after a few echoes, in practice about ten echoes. To an even greater extent, this component of the signal is no longer measurable when 128 successive echo pulses are implemented in a practical sequence.
Conditions known as the Carr-Purcell or CP conditions have also been proposed. In these conditions, the flip and refocusing excitations are located on a same axis (for example the axis y). And the flip and first refocusing excitations are in phase. In this case, from one refocusing excitation to another, the direction of the refocusing excitation is changed (they are in phase or in phase opposition with this first refocusing excitation). The demodulation processing is adapted. It can easily be shown, especially with reference to FIG. 1g, that we are then in a situation comparable to that of FIG. 1f (the one showing the positive effects of the CPMG conditions): in any case, an imaginary or real component of the NMR signal is destroyed.
In the patent cited above, this problem is overcome by modifying the phase of the 180° refocusing pulse (and therefore that of the receiver at the time of the reception). This modification is done quadratically from one refocusing pulse to another. In other words, the phase Φ of each refocusing pulse is of the Φ=Δi2 type in which Δ is a sweep factor and in which i is the number of the 180° refocusing pulse or 180° pseudo-refocusing pulse.
This means that it all amounts to ensuring that the 180° refocusing pulses or the imperfect 180° refocusing pulses do not act identically on all the same magnetic moments to the point of destroying the imaginary component of some of them. In the above-mentioned patent, provides for a gradual sliding of the phase in a quadratic progression. This quadratic progression is equivalent to a linear sweep in frequency, which can be likened, for its part, to a shifting of the date of reading on the read axis (typically the above axis x). In other words, according to the patent cited above, from one refocusing excitation to another focusing excitation, the phase varies quadratically.
When the method with quadratic phase shift was implemented, it was realized nevertheless that the state of dynamic equilibrium was not immediately established. Consequently, there was a loss in signal level. Indeed, there was a residual oscillation of the measured signals, resulting in a loss of sensitivity. To prevent this oscillation, dynamic equilibrium thus prompted can be stabilized as soon as the first echoes occurred. This stabilization was obtained by subjecting the magnetizations to an initial preparation during a limited number of echoes. The preparatory echoes may furthermore be used for the reconstruction of the images. The number of preparation echoes was limited to seven. A better (but only marginally better) result can be obtained by choosing a greater number of preparation echoes.
To be able to determine the characteristics of these preparatory echo pulses, a procedure of successive images was then initiated. This entailed the exploration, with small increments, of the effects of the different phase shifts of the refocusing pulses on each other. According to a complex theory that has to be explained, it was foreseen that the phenomenon would accept a stationary characteristic, for which the eigen vectors (or eigen functions) were sought. With these eigen functions, a mode of preparation for this dynamic equilibrium was determined so that ultimate measured signal level was the greatest possible level. It was then found that, provided the phases of the first refocusing pulses were thus conditioned in a special way, a better result was obtained. The preparation depends on the chosen law of quadratic progression and on the amplitude of the initial echo excitation.